A conventional lamp inlead assembly which can be utilized for instance in a high pressure discharge lamp or a halogen filament lamp, typically includes an outer lead wire, a thin molybdenum foil and an inner lead wire. This assembly can be inserted into a double ended lamp envelope which is then pressed, or shrunk over the inlead assembly to provide a hermetic seal for the inner chamber of the lamp envelope. Typically, the molybdenum foil is extremely thin, being on the order of no more than 3 mills thick with feathered edges on either side. Though the feathered edges are effective for allowing a more complete sealing operation to be performed at the ends of the lamp envelope, such feathered edges are susceptible to damage when handled during the lamp assembly process particularly if a high speed automated manufacturing process is utilized for assembly of the lamp or the lamp inlead assembly.
One approach to avoid the handling problem for thin molybdenum foils can be found in U.S. Pat. No. 4,254,356 issued to Karikas on Mar. 3, 1991 and assigned to the same assignee as the present invention. In this patent, a Z-shaped foil configuration is utilized wherein the edges of the foil are turned outward in opposite directions thereby rendering the foil member more rigid and less susceptible to damage from indelicate handling. This approach has proven effective not only in strengthening the foil member but has done so without suffering from a reduction in the quality of the seal formed by shrinking or pressing the end of the lamp envelope around the lamp inlead assembly.
One problem that has remained however in the use of such a Z-shaped foil configuration relates to the ability to maintain the control of the positioning between the lead wires and the foil member just prior to and/or just after attachment of the lead wires to the foil member. That is, because the center portion of the Z-shaped foil member is essentially flat, maintaining the essentially round lead wire in a precise position for first welding or brazing to the foil member and then placing the entire assembly essentially centrally within the lamp envelope end prior to sealing, has proven to be difficult. For optimum lamp performance, it is essential that the inner lead wire reside on center of the lamp envelope. A lead wire that is either off center relative to the foil member or is at an angle relative to the edges of the foil member, can result in a poor seal around the lamp inlead assembly or can put stress on the connection between the lead wire and foil member such that early failure of the lamp would occur. Additionally, such a positioning error would result in electrode misalignment in a discharge lamp or an off-center filament for a Halogen-IR lamp, both of which conditions are to be avoided. Another problem associated with the Z-shaped configuration is in the manufacturing operation associated with inserting this lamp inlead assembly into a lamp envelope. For the Z-shaped arrangement, the practice has been to have the top and bottom portions of the foil member contact the inner surface of the end region of the lamp envelope. Such a contacting operation inherently slows down the manufacturing process and is less efficient than one where there was a clearance provided between the foil member and the surface of the lamp envelope.
Other approaches to solving the problem of the delicate handling requirement of the foil member so that a precise and secure seal can be formed there around, can be found in U.S. Pat. Nos. 3,582,704; 4,136,298; and, 4,851,733. In each of these patents, a foil member is utilized which has a non-uniform thickness associated therewith. In other words, the foil members have various shapes, thicknesses and various gradations in thickness to provide stability, strength and sufficient foil member dimension to insure adequate current carrying capabilities. The problem with such a variation in the thickness and shape of the foil however is that the sealing process for sealing the end of the lamp envelope around the inlead assembly containing the thicker foil member is adversely affected by the increased thickness of the foil member and the quality of such seal may suffer accordingly.
It would therefore be advantageous if an inlead assembly could be provided that would include a foil member that was thin enough to insure a proper, high quality seal at the end of the lamp envelope, could achieve such high quality seal even when utilized in a high speed automated manufacturing environment and further, could allow that the positioning between the lead wires and the foil be accurately maintained throughout the lamp manufacturing process. It would be further advantageous if such a lamp inlead assembly could accomplish all of these features while at the same time, insuring that a sufficient foil dimension control is achieved to handle the necessary current while providing additional space for clearance of the foil member during lamp manufacture.